Several attempts have been made to reduce the contact stress in gear teeth, since a high contact stress causes pitting and eventually failure of the teeth. Many of these attempts have been based on the Wildhaber or Novikov types of gearing, in addition to the more common involute forms. In Novikov gears, the profiles in the transverse sections are circular arcs, the pinion being convex and the gear being concave. The problem with circular arc profiles is that they are not conjugate. With conjugate profiles, the angular velocity ratio is exactly constant. In each transverse section there is continuous contact throughout the meshing cycle, and the contact point moves along each profile, towards the tip of the driving tooth and towards the root of the driven tooth.
Since a variable angular velocity ratio is not acceptable, Novikov gears are always made helical. This means that in each transverse section, only one pair of points ever comes into contact. At any instant, there will be one transverse section where contact occurs, and this contact point moves axially along the tooth face as the gears rotate. Such gears are generally noisy, and have not been widely used. Moreover, the same comments apply to Wildhaber gears, which are shaped as circular arcs in the normal sections.
U.S. Pat. No. 6,101,892 describes a major advancement in gear tooth profile design that greatly improves the load carrying capability of a parallel axis helical gear pair. Due to the fact that the stress optimized dedendum and addendum of this technology—both of these contact areas conformal in nature, (that is one of the surfaces is convex and the other is concave) versus convex/convex for the classical forms of involute pair—the area in and around the pitch point where the reversal of these curvatures takes place from concave (dedendum) to convex (addendum) presents several challenges.
Accordingly the 892 patent teaches a transition zone between the concave dedendum to the convex addendum in which no tooth contact occurs. FIGS. 1 and 1A (Prior Art) illustrate the transverse section profiles of a 15-tooth pinion and a 90-tooth gear, and FIG. 1A shows a prior gear pair having no contact along the transition zones 20a and 22a. It is important to note that due to the scale of FIGS. 1 and 1A, and the slight and subtle nature of the transition zone it is impossible to discern simply by observing FIG. 1 the fact that no contact is made, at the flanks which are in contact, along the transition zone. This no contact condition is shown in FIG. 1A. Transition zones 20a and 20b of the right-most teeth of gears 20 and 22 are shown spaced apart (i.e. not in contact). The addendum 20c and 22c, and dedendum 20b and 22b portions of these profiles are designed according to the teachings of the 892 patent.
The transition of an active flank (that is, one that contacts the opposing gear tooth) into and out of the transition zone (that is, where there is no contact has been achieved in prior art configurations as smooth and continuous slopes. Abrupt changes in curvatures of these joining features can precipitate undesirable surface characteristics such as micropitting. U.S. Pat. No. 6,964,210, entitled “Gear Tooth Profile,” and U.S. Pat. No. 7,077,026, entitled “Gear Tooth Profile Curvature,” disclose a transition zone in which there is no contact.